The present invention relates to an organic electroluminescent element.
An organic electroluminescent element (or an organic EL element) includes an anode, an organic electroluminescent layer (or an organic EL layer) and a cathode, and these are layered and generally formed on a substrate. A component on a light output side relative to the organic EL layer has a transparency (to be transparent) against light outputting outside the organic EL element. The organic EL element emits light due to electric current flowing through the organic EL layer as electric voltage is applied between the anode and the cathode.
Such an organic EL element should reduce its volume and area occupied by the element and the wiring connected to the element, since the size of a device (for example, a portable terminal and the like) to which the element is assembled is limited. The organic EL element also should increase its efficient display region (light-emitting area) for the same volume and area. Furthermore, the organic EL element should simplify connections between the organic EL element and an external drive circuit for driving the element so as to simplify manufacturing.
For the above requirements, an anode terminal and a cathode terminal were conventionally arranged on one side of the organic EL element, as disclosed in columns [0005], [0006], [0009], and FIGS. 1 and 3 of Unexamined Japanese Patent Publication No. 7-211459. Such organic EL element will now be described with reference to FIGS. 7A and 7B.
The above conventional organic EL element 21 partially includes a transparent electrode (anode) 23 made of indium tin oxide (“ITO”) formed on a substrate (transparent substrate) 22 having an extended portion 221. A collector 24 is formed on the transparent electrode 23 so as to double as an externally leading terminal (anode terminal) 25 in a state where it extends toward the extended portion 221 of the substrate 22. A luminescent layer (organic EL layer) 26 and a reflective insulating layer (not shown) are sequentially formed on the transparent electrode 23 and the collector 24 (except for the externally leading terminal 25). A backside electrode (cathode) 27 is formed on the luminescent layer 26 and the reflective insulating layer. The backside electrode 27 is formed to extend toward the extended portion 221 of the substrate 22 so as to partially double as an externally leading terminal (cathode terminal) 25′.
Such an organic EL element 21 is reduced in size by the width of A in FIG. 8, that is, the width of one terminal, in comparison to a structure in which terminals of respective electrodes are provided on opposite sides of the rectangular organic EL element 21 as shown in FIG. 8.
In comparison to the organic EL element 21 shown in FIG. 8, the substrate having the same size allows a larger-size (area) organic EL layer to be arranged thereon. Specifically, even a region where the terminal 25′ is arranged may be used as an efficient display region, that is, may be provided with an organic EL layer.
Furthermore, since the externally leading terminals are provided at one side of the organic EL element 21 in FIG. 7B, which is different from that of the organic EL element 21 of FIG. 8, wiring work is performed only on one side of the organic EL element 21. Accordingly, it is possible that a connector and the like is provided for connecting the anode terminal 25 with the cathode terminal 25′ shown in FIG. 7B and only plugged to these terminals to finish the wiring work. Additionally, area and volume occupied by wiring in a portable terminal between an external drive circuit and the organic EL element 21 may be smaller than the element 21 of FIG. 8.
However, as the terminals are arranged at one side of the luminescent region, there occurs a problem that the intensity of electric current flowing at each spot of the organic EL layer is relatively large on a side near the terminals and relatively small on a side far from the terminals. This problem occurs because at least one of the anode and cathode increases its volume resistivity to a measurable extent.
As described above, the organic EL element needs to output light from at least one side relative to the organic EL layer. Accordingly, the electrode on the light output side should have a transparency against the above output light and should also have a characteristic of not degrading the organic EL layer, which thereby limits materials to be employed. Therefore, generally, the electrode on the light output side should be made of a material having a high volume resistivity in comparison to the electrode on the other side.
For the above reasons, a path leading to a cathode luminescent region through the organic EL layer adjacent to the anode terminal has a smaller resistance value, while a path leading to a cathode luminescent region remote from the anode terminal has a larger resistance value. In other words, electric current flowing at each spot of the organic EL layer decreases as it distances from the anode terminal. Therefore, there have been the following problems.
Generation of Unevenness in Brightness
Since the organic EL element has a portion of relatively large electric current flow and a portion of relatively small electric current flow, unevenness in brightness is generated in the element as a whole. As the electric current flow increases, the brightness of the organic EL element rises. Therefore, as the organic EL element has a portion of relatively large electric current flow and a portion of relatively small electric current flow, brightness differential arises in the element between the above portions, which generates unevenness in brightness.
Generation of Life differential in the Element
The life of the element varies between the portion of relatively large electric current flow and the portion of relatively small electric current flow. Generally, the life shortens at the portion of relatively large electric current flow. In comparison to an organic EL element having uniform electric current flow, this organic EL element has a portion having a relatively short life, which shortens the life thereof. Additionally, when the element is used for a long period, there arises a portion not emitting light or a lower brightness portion than the other portions.
Problems such as Degradation
Since the organic EL element has a portion of relatively large electric current flow and a portion of small electric current flow, there may occur degradation by location.
Generation of Unevenness in Chromaticity
Since the organic EL element has a portion of relatively large electric current flow and a portion of small electric current flow, it causes S-S annihilation phenomenon in the organic EL element including a fluorescent material and also causes T-T annihilation phenomenon in the organic EL element including a phosphorescent material. Accordingly, in an organic EL element containing a plurality of luminescent materials in the luminescent layer for emitting colors of different wavelengths from at least one other luminescent material, brightness of each layer may differ between a portion of relatively smooth electric current flow and a portion of relatively unsmooth electric current flow, with a consequence of possible generation of unevenness in chromaticity.
Accordingly, when the terminals are merely provided on one side of the display area, the above mentioned problems occur, so that it has been difficult to actually use the above structure as an organic EL element. Therefore, there is a need for providing an organic EL element having terminals on one side of respective luminescent regions and small differential in intensity of electric current flowing at each spot of the organic EL layer. Also, when the above organic EL element is provided, the inventors of the present application found appropriate correlation between the terminals and the luminescent regions for reducing differential in intensity of electric current flowing at each spot of the organic EL layer.